Numerical simulation and laboratory experiments of CO2 sequestration and being as cushion gas in underground natural gas storage reservoirs

Underground natural gas storage reservoir is generally used to stabilize the natural gas supply to meet the high and low peak of gas consumption. In addition, in the case of depleted gas reservoirs, the leftover native gas or cheap inert gas being as the cushion gas is needed to maintain reservoir pressure and ensure the stability of gas storage. In recent years, injecting CO2 for use as the cushion gas has drawn great attention for its potential to sequestrate the greenhouse gas underground and improve the operating efficiency of the working gas. In this study, a three-dimensional component model coupled with convection and diffusion is firstly constructed to describe the flow and miscible displacement processes affected by mixing in the porous media surrounding storage wells, and a fully implicit numerical model is established based on unstructured tri-prism meshes and a hybrid control volume finite element (CVFE) and finite-difference (FD) method. Secondly, mixing experiments of CO2 as cushion gas are carried out, the results are verified by numerical model, and good agreement between measured and computed data is achieved. Finally, through the numerical simulation of the hypothetical cases and orthogonal experiments, it is can be concluded that: CO2 is more suitable as cushion gas than natural gas itself and N-2, injecting CO2 through corner wells at the bottom layer can effectively prevent the working gas escaping, and help to store more CO2 and slow down the mixing zone runs up, and the high production rate will help to control the mixing zone at a lower position and obtain greater gas recovery.